Allow users to specify the redirect URI

[poblouin]

Is your feature request related to a problem? Please describe:
When a user uses a custom redirect URI for the auth code grant (other than https://example.com), that result in an Invalid Redirect URI error.

Describe the solution you'd like:
Instead of hardcoding the URI, allow users to pass custom URI in the plugin's config. By default we will use example dot com and if the config field is populated, we will use this one.